The electrochemical sensors can be divided into those utilizing high temperature nonporous, inorganic electrolytes and those using gas permeable low temperature electrolytes. Of the latter category a further division can be made according to whether the electrolyte is a solid polymer on one hand, or a liquid, paste or gel on the other.
A typical solid polymer electrolyte (SPE) based electrochemical sensor has at least a working (sometimes referred to as a sensing electrode) and counter electrodes in contact with an SPE. A reference electrode may also be used to form a 3-electrode device. The electrode material, preferably a metal with catalytic properties, is typically Au, Pt, Pd or noble metals and their alloys in the form of wire grids, powders or films.
Various structures and means have been used to apply or otherwise bring a catalyst in contact with an electrolyte to form working, counter, and reference electrodes. The electrode membrane constructions can be summarized as: (a) solid metal films in contact with the SPE, (b) porous metal films or planar distributions of metal particles deposited on top of the SPE or powders pressed against an SPE surface, (c) metal grids or meshes deposited on top of the SPE or embedded within an SPE layer, or (d) separate sheets of catalyst particles bonded in Teflon, which are pressed against the SPE.
Examples of such an known electrode membrane constructions include:
(1) metal (Pt) films formed by electroless chemical plating directly onto Nation.TM. membrane surfaces; PA1 (2) porous Pt layers chemically plated onto a Nation.TM. film, forming a network of interconnected islands of Pt particles interspersed with roughly circular regions (40-100 .mu.m diameter) of Nation.TM. film containing little or no platinum; and PA1 (3) Pt discs spin coated with Nation.TM. solution followed by electrodeposited ruthenium ad-atoms onto a Nation.TM. film. PA1 (1) membrane electrodes with a conducting layer of contiguous metal particles deposited by vacuum evaporation on, and adhering to a non-porous material, overcoated with a permeable membrane layer; PA1 (2) porous 100 nm thick Au films vacuum vapor deposited on to Nation.TM. films; PA1 (3) grid electrode membranes has been produced by vacuum evaporation of Au onto Nation.TM. substrates through photolithographically etched masks, followed by spin coating Nation.TM. solutions over such electrode configurations; the length-to-width ratio of the grid wires being varied; PA1 (4) thin film electrode Nation.TM. film structures constructed by sputtering Pt through photolithographically etched masks onto various substrates and overcoating the Pt electrodes with various Nation.TM. films coated from solution; PA1 (5) ultrafine grid structures made by photolithographically etching 30 nm thick Au films previously deposited on oxidized Si wafers, and after etching, spin coating with Nation.TM. solutions; and PA1 (6) multiple electrodes, biased to null out environmental effects, formed on a common substrate by depositing a metal film and overcoating with a Nation.TM. membrane. PA1 (1) Pt and Ag powders pressed to the sides of SPEs made of compacted discs of Teflon.TM. and zirconium phosphate powders and antimonic oxide powders; PA1 (2) metal powders pressed into Nation.TM. sheet surfaces, and contacted with a gold mesh; PA1 (3) catalyzed carbon black loaded into the surface of Nation.TM. membranes and placed in contact with carbon black loaded Teflon membranes; PA1 (4) a gold minigrid (500 wires/inch) mechanically pressed into the surface of a Nation.TM. film, followed by "gluing" the pressed minigrid to the membrane by solution casting a further layer of Nation.TM. solution over the assembly since the mechanical pressing tends not to give good contact by itself; and PA1 (5) Pt wire meshes partially hot pressed into a Nation.TM. surface. PA1 (1) electrodes comprised of hydrophobic Teflon.TM.-bonded Pt black layers pressed against Nation.TM. membranes; and PA1 (2) electrodes pressed against Nation.TM. membranes, the electrodes being fabricated from platinoid black and Pt-5% Ir catalyst compositions blended with a Teflon.TM. binder. PA1 "acicular" means having an aspect ratio of.gtoreq.3; PA1 "aspect ratio" means a ratio of an element's height to its average cross-sectional width; PA1 "discrete" means distinct elements, having a separate identity, but does not preclude elements from being in contact with one another; PA1 "nanostructured element" means an acicular, discrete, oriented, sub-microscopic, preferably a two-component structure comprised of a whisker coated with an electrically conductive material; alternatively, the nanostructured element may be a one-component structure wherein the electrically conductive material only forms the discrete, oriented structure; PA1 "nanostructured electrode membrane" means a film containing nanostructured elements in an encapsulant, wherein the encapsulant is an electrolyte-containing polymer and the membrane may be configured for either a two-electrode or three-electrode sensor; PA1 "oriented" includes random or uniaxial; PA1 "solid electrolyte" includes non polymeric materials of a solid consistency that will allow ionic conductivity; PA1 "solid polymer electrolyte" includes polymer materials of a solid consistency that will allow ionic conductivity; PA1 "submicroscopic" means having at least one dimension smaller than approximately a micrometer; and PA1 "whisker" means the inert core of the nanostructured element.
Examples of metal grid or mesh based electrodes include:
Examples of electrode membranes wherein catalyst particles are pressed onto an SPE include:
Other examples of electrode membranes include:
Processes for bonding a catalytic material coated onto a current collecting screen and embedding it into the surface of a polymeric cation exchange membrane and other basic processes and properties of an electrode formed of a mixture of noble metal particles bonded with hydrophobic materials have been described.
While the art known electrode membranes have proven useful, such electrode membranes constructed from vapor coated grids and metal films tend to suffer several severe mechanical problems, delamination and cracking as a result of swelling and shrinkage of the SPE, especially when exposed to humidity. Such problems contribute to a decrease in signal from the electrodes with time over and above changes due to catalytic site effects. The adhesion of vapor deposited noble metal films to polymers tends to be poor, thus requiring an adhesion promoting layer like Cr to be deposited first, which can lead to corrosion and degradation of the electrode membrane upon use. Furthermore, the pressed metal meshes generally suffer delamination problems as known in the art.